docs: fix verbatim in RST

qiskit_nature/second_q/properties/angular_momentum.py

@@ -59,7 +59,7 @@ def __init__(self, num_spatial_orbitals: int, overlap: np.ndarray | None = None)
         Args:
             num_spatial_orbitals: the number of spatial orbitals in the system.
             overlap: the overlap-matrix between the $\alpha$- and $\beta$-spin orbitals. When this
-                is `None`, the overlap-matrix is assumed to be identity.
+                is ``None``, the overlap-matrix is assumed to be identity.
         """
         self.num_spatial_orbitals = num_spatial_orbitals
         self._overlap: np.ndarray | None = None